In order to study the functional role of substance P in substantia nigra, we have studied whether the content of substance P in nigra changes when the function of dopamine neurons is perturbed. Chronic treatment of rats with haloperidol results in a significant decrease of substance P concentration in substantia nigra; in contrast, the substance P content of other brain regions such as striatum and hypothalamus was not affected. This finding suggests that dopamine neurons either directly or indirectly may be involved in regulating substance P content in the substania nigra. A decrease in the substance P content of substantia nigra occurred only after repeated haloperidol injection, but not after a synthetic capability of the system. Perhaps a large pool of substance P precursor may be operative. The chick embryo dorsal root ganglion contains a molecularly heterogeneous pool of substance P-like peptides including a high molecular weight species which could function as a precursor in the formation of substance P. The content of this possible precursor is regulated by treatment of ganglia with nerve growth factor. Substance P, and its apparent precursor have also been found in superior cervical ganglia, probably located in interneurons.